Taste buds consist of numerous elongate modified epithelial cells, some of which serve as the transducing elements for taste. Different taste cells are involved in transduction of different taste qualities. Experiments in this R21 proposal are designed to generate and test a transgenic mouse in which under the influence of cre-recombinase, synaptically-connected taste cells will produce the transneuronal tracer, Wheatgerm Agglutinin (WGA). The WGA should cross the synapse into the gustatory nerve fibers thereby labeling ganglion cells and perhaps terminals within the nucleus of the solitary tract. If sufficient WGA is transported, the tracer may also label postsynaptic cells in the nucleus of the solitary tract. The construct to be employed for generation of the mouse is a knock-in whereby the BDNF coding region is flanked by Iox sites and followed by the WGA coding sequence. When acted upon by cre-recombinase, the BDNF coding region is excised and WGA is produced instead. We will use various existing transgenic lines to express crerecombinase in relevant BDNF-expressing populations in cranial ganglia and the brainstem as well as in taste buds. For selective recombination in taste buds, we will utilize an existing K14tamcre line where crerecombinase can function only in keratin 14-expressing cells under the influence of tamoxifen, thus providing us with good spatial and temporal control. Since basal cells of taste buds express keratin 14, tamoxifen treatment will activate cre-recombinase thereby resulting in production of WGA by taste cells that would have produced BDNF. The population of taste cells that normally express BDNF are synaptically-connected cells that include those specifically implicated in "sour" (H'+)transduction. Thus the WGA-expressing transgenic mice that we propose will specifically mark taste cells and ganglion cells involved in "sour" transduction. These mice also will be informative about the kinetics of protein handling, exocytosis and uptake by various cell types within the taste bud.